Regulation of Starch Biosynthesis in Response to Environment
Starch Biosynthesis Starch is synthesized inside plastids in plants. First ADP-Glc pyrophosphorylase (AGPase) catalyzes the conversion of Glc-1-P and ATP to ADP-Glc and inorganic pyrophosphate (PPi). ADP-Glc donates glucosyl for starch synthesis, elongating the a-1,4-linked glucan chains of amylose and amylopectin, the two insoluble starch polymers the compound consists of. Amylose is relatively linear and makes up 20-30% of normal starch while amylopectin is much more branched and makes up the rest of the composition of the molecule. Starch branching enzymes (SBE I and SBE II) introduce branch points into the polymer chains by cleaving and tailor the glucans into forms capable of the proper crystallization of the starch molecule. Regulation of starch biosynthesis acts on AGPase, the catalyzing enzyme responsible for its production. Regulation of AGPase AGPase is constructed of two small (APS; 50 kD) and two larger (APL; 51 kD) subunits, all of which are required to optimal enzymatic function. There are multiple genes encoding for the subunits of AGPase and much evidence that the expression of these genes is spatially and temporally controlled. The expression of AGPase is increased by the presence of sugars and decreased by the presence of nitrate and phosphate, allowing for the accumulation of starch in response to changes in environmental limitations as well as changes in carbon and nutrient levels. The regulation of ATPase occurs in several different ways, the first of which being allosterically. Activity of AGPase is activated by glycerinate-3-phosphate (3PGA) and inhibited by Pi. The increase in levels of phosphorylated intermediates such as ATP show an increase in the 3PGA-Pi ratio. An increasing 3PGA-Pi ratio will show an increase in the activation of AGPase and thus an increase in the rate of starch synthesis. Another method of regulation occurs after translation of the enzyme. AGPase can be affected by a post-translational redox regulation that can reverse the formation of the intermolecular Cys bridge formed between Cys-82 on each of the smaller subunits of the enzyme. Reduced forms of the enzyme displays the two subunits as separate monomers while the oxidation of the enzyme results in the subunits as a dimer that displays lower activity. Reduction of the AGPase alters the kinetic properties of the enzyme, resulting in an increase in AGPases sensitivity to activation by 3PGA and a decrease in the enzymes sensitivity to inhibition by Pi. Regulation of AGPase in Response to Light and Sugar Signals Starch is synthesized in the chloroplast of leaves during the day and degraded at night, this is controlled by two different mechanisms acting on AGPase to allow synthesis of starch in the light and degradation of starch in the night. The first is the illumination of leaves during the day that leads to fast redox activation of AGPase, while at night in the absence of light, this process is completely reversed. The second is the allosteric regulation of AGPase already discussed. Because 3PGA is the primary fixation product of the Calvin-Benson cycle, its concentration is increased during the day when light is activating the cycle. The increasing 3PGA will increase the activation of AGPase and therefore increase the synthesis of starch. The amount of carbon contained in leaves fluctuates drastically with the modification of the rate of photosynthesis due to changes in light. This is corrected for by the accumulation and use of starch as a carbon reserve when primary carbon levels are low. When light intensity decreases, sugar reserves are depleted which leads to a temporary halt in the utilization of carbon for growth. Once this has happened, sugars begin to accumulate and starch biosynthesis is initiated. The regulation of transcription for the enzyme AGPase is essential to plant life. Although the starch AGPase synthesizes is necessary for the life of plants, having it in full production all of the time would waste valuable energy and resources. By allowing the enzymatic activities of AGPase to increase and decrease in response to environmental stimuli, the plant no only conserves energy but builds up reserves of essential nutrients needed for its survival. References Geigenberger, P. (2011). Regulation of Starch Biosynthesis in Response to a Fluctuation Environment Plant PHysiology ''(155). Tetlow, I. a. E. M. (2014). A review of starch-branching enzymes and their role in amylopectin biosynthesis. ''IUBMB Life, 66(8), 546-548.